In railway signaling, axle counters are used among other things to monitor track sections. Each axle counter contains counting points with two rail contacts and one or more evaluation units.
Each axle counter monitors a track section assigned to it. If the axle counter detects a rail vehicle passing, the track section is switched to occupied. If the next axle counter in the direction of travel of the rail vehicle detects the passing rail vehicle, the track section is switched to free again (track release).
When a vehicle wheel passes over, two adjacent rail contacts are actuated one after another and two pulses that overlap in time are triggered. These pulses are evaluated in the evaluation unit with regard to their amplitude and are converted into counting pulses, the sequence of pulses produced by the direction of travel of the passing vehicle axles determining the respective counting direction of the pulses.
Electronic rail contacts often comprise two transmitting heads with transmitting coils mounted on a rail and lying spatially one behind the other, which heads are supplied with audio-frequency alternating currents, and two receiving coils of receiving heads arranged on the respectively opposite rail side and coupled inductively to the transmitting coils. One transmitting and one receiving coil respectively together form a pulse generator. The voltages induced in the receiving coils are supplied to an evaluation unit with a receiving circuit arranged in the vicinity of the rail contact and evaluated there. The temporary drop and the phase rotation of the voltages induced in the receiving coils are evaluated as an indication of the passing of a vehicle wheel at a rail contact. The drop and the phase rotation of the receiving voltages are determined by the coupling between the transmitting and receiving coils when a vehicle wheel passes. The voltages induced in the receiving coils are converted via an output voltage of the receiving circuit generated in the receiving circuit into digital signals, from which counting pulses dependent on the direction of travel are finally derived.
It is a prerequisite for proper operation of the axle counting systems controlled by the electronic rail contacts that the output voltages sent by the receiving coils to the evaluation unit and determined from the receiving voltages are not also dependent in the amplitudes on parameters that have nothing to do with the influence of the vehicle wheels. Such influences can for example be due to temperature variations in the transmitting coil and thus to a temperature-dependent receiving voltage. Furthermore, the output voltage must lie in predetermined value ranges, so that the evaluation unit can evaluate the output voltage correctly. The receiving voltage induced in the receiving coil, however, is determined strongly by the magnetic properties of the surroundings of the axle counter. If the place of use of the axle counter is e.g. on a railway bridge made of ferrous metals, the receiving voltage is substantially greater than in the case of a place of use in a track area with a track bed of gravel. The receiving voltage can vary by several 100% due to such influencing factors.
To keep the output voltage in the predetermined value range, an adjustment of the receiving voltage is undertaken. This adjustment can be carried out e.g. mechanically, the mechanical construction of the axle counter being varied in such a way that the receiving voltage corresponds to a desired value. An electrical adjustment can also be carried out. This is safer and more convenient on account of the fact that no work on the track is necessary for this. Furthermore, in the case of an electrical adjustment the mechanical construction of the axle counter can be simpler, sturdier and cheaper due to the elimination of the adjustment console. In an electrical adjustment according to the prior art, an electric current, which is obtained from a digital signal and is in phase opposition to the electric current flowing through the transmitting coil, is injected into a branch of the receiving circuit. An adjustment voltage corresponding to the current in phase opposition is thereby taken from the receiving voltage, so that when a wheel passes through, a change of sign of the output voltage occurs, which is interpreted as a wheel counting pulse. The receiving circuit has a current sensor transformer, which taps the receiving voltage via its primary winding from an oscillating circuit of the receiving head. The current sensor transformer has two secondary windings, via which a first partial receiving voltage and a second partial receiving voltage of opposing polarity to the first partial receiving voltage are generated in a branch respectively of the receiving circuit. These two partial receiving voltages are superposed via a differential amplifier circuit, e.g. the formation of a difference takes place. The electrical adjustment is carried out such that if no vehicle wheel passes an output voltage of e.g. 200 mV is supplied. If a vehicle wheel passes the rail contact, a current in phase opposition is induced in the receiving head, or its receiving coil. The then resulting output voltage is then minus 200 mV. This voltage change is registered in the evaluation unit as a wheel passing.
The electric current obtained from the digital signal and injected into at least one of the branches, and the corresponding adjustment voltage superposed with the partial receiving voltages is, seen in itself, temperature-stable. Due to the injection in phase opposition, this constant current is deducted from the temperature-dependent input signal of the receiving circuit, i.e. from the temperature-dependent partial receiving voltages. The resulting difference, i.e. the resulting output voltage of the receiving circuit, is then assigned a markedly higher temperature coefficient than an output voltage that is not electrically adjusted.
A strong temperature dependence of the output voltage of the receiving circuit thus adjusted results from this. This strong temperature dependence prevents a general usage of this electrical adjustment.
Temperature variations in the transmitting coil can have a negative effect on the mode of operation of the rail contacts. This can be the case in particular if the transmitting power, due to an electric current flowing through the transmitting coil that is dependent on the temperature of the transmitting coil, is subject to uncontrolled variations. The adjusted output voltage can in this case vary in such a sharply temperature-dependent manner that any evaluation by the evaluation unit is no longer possible. Considerable safety problems can arise from this when using the rail contacts in railway operation due to incorrect assessment of the receiving signal of the receiving coil.